1. Field of the Invention
The present invention relates to a phase locked loop (PLL) and, more particularly, to a PLL with phase rotation for spreading spectrum.
2. Description of Related Art
With rapidly improved electronic technologies, various applications can be provided by the electronic products. However, due to the increasingly complex electronic applications, the clocks required for the applications are different, and accordingly a PLL becomes an optimal choice to meet the requirements of saving the cost and providing the different clocks. FIG. 1 is a block diagram of a typical PLL 100. As shown in FIG. 1, the PLL 100 includes a phase detector 110, a charge pump 120, a low pass filter (LPF) 130, a voltage controlled oscillator (VCO) 140 and a frequency divider 150. The PLL 100 receives an input signal with a frequency Fin and produces an output signal with a frequency Fout, for Fout=Y×Fin and Y is an integer or a fraction. In this case, the electromagnetic interference (EMI) detection cannot be passed frequently because the frequency Fout of the output signal is relatively greater than the frequency Fin of the input signal. Also, the other electronic components can be easily interfered.
To overcome the aforementioned problem, U.S. Pat. No. 6,377,646 granted to Sha for a “Spread Spectrum at Phase Lock Loop Feedback Path” discloses a method for spreading spectrum of a PLL output signal, which uses a read only memory (ROM) to record a correction value for changing the divisor currently used in the feedback divider and applies a phase swallowing to the feedback divider for spreading spectrum of the PLL output signal. However, such a spreading way essentially changes the divisor in the feedback divider to thereby change the output frequency, and cannot be applied to a PLL in which the output frequency necessarily equals to the input frequency. In addition, such a spreading way cannot be applied when a feedback divider has a relatively small divisor, which causes the phase detector to output a significant error signal and further affects the spreading effect.
Therefore, it is desirable to provide an improved PLL to mitigate and/or obviate the aforementioned problems.